The present invention relates to a dispersion-shifted single-mode optical fiber suitable for being used in wavelength division multiplexed transmission systems having high data rates.
Single-mode fibers must have characteristics that correspond to the requirements both of cable-makers and of system designers: firstly they must have good xe2x80x9ccablabilityxe2x80x9d, i.e. putting the fiber in a cable must give rise to only very low additional attenuation; and secondly they must have large effective areas to avoid non-linear effects, and suitable values for the zero-dispersion wavelength xcex0. 
The use of such fibers in wavelength division multiplexed transmission systems using RZ, NRZ, or soliton-type pulses involves further constraints, particularly with increasing number of transmitted channels, data rate per channel, and post-amplifier power, and with decreasing spacing between channels. It is preferable to have a fiber that has non-zero chromatic dispersion at the transmission wavelengths so as to avoid non-linear effects, and in particular four-wave mixing (FWM) in the presence of a plurality of channels. Non-zero dispersion-shifted fibers (NZ-DSFs) are therefore used that have zero chromatic dispersion wavelengths xcex0 that lie outside the ranges of the channels of the multiplexes so as to avoid the problems caused by four-wave mixing.
In addition, the use of such fibers for wavelength division multiplexed systems involves looking for a shallow chromatic dispersion gradient so as to retain propagation characteristics that are similar for the various channels.
Numerous index profiles have been proposed for such dispersion-shifted single-mode optical fibers. The index profile is generally described as a function of the appearance of the curve representing the refractive index as a function of the radius of the fiber. Conventionally, distance r to the center of the fiber is plotted along the x-axis, and index, defined by its difference relative to the index of the cladding of the fiber, is plotted up the y-axis in relative value (xcex94n) or in percentage (xcex94n%=100xc3x97xcex94n/n).
The index profile is thus said to be xe2x80x9csteppedxe2x80x9d, xe2x80x9ctrapezium-shapedxe2x80x9d or xe2x80x9ctriangularxe2x80x9d for curves representing the variation of refractive index as a function of radius that are respectively stepped, trapezium-shaped, or triangular.
The article entitled xe2x80x9cNew dispersion shifted fiber with an effective area larger than 90 xcexcm2xe2x80x9d, Proceedings of the European Conference on Optical Communication, vol. 1, Sep. 15-19, 1996, pages 49-52, describes, inter alia, reference profiles of the xe2x80x9ccentral troughxe2x80x9d type.
The fiber profiles proposed in that article have a zero chromatic dispersion wavelength lying in the range 1572 nm to 1583 nm, with an effective area in the vicinity of 90 xcexcm2. Those two characteristics make it possible to satisfy the requirements in terms of non-linear effects and of four-wave mixing. However, such fibers cannot be used in high data rate tele-communications systems because, at 1550 nm, they have chromatic dispersion that is too low, and in practice lower in absolute terms that 2 ps/nm.km.
An object of the present invention is thus to develop a dispersion-shifted optical fiber that is optimized for high data rates, i.e. that has a large effective area, low losses, and a zero chromatic dispersion wavelength that is distinct from 1550 nm, making it possible to avoid the four-wave mixing phenomenon.
To this end, the present invention provides a dispersion-shifted single-mode optical fiber having an effective area greater than or equal to 60 xcexcm2;
characterized in that it has a zero chromatic dispersion wavelength xcex0 that is greater than 1585 nm and chromatic dispersion at 1550 nm that is greater in absolute terms than 3 ps/nm.km.
The fiber of the invention is a dispersion-shifted single-mode fiber having a large effective area, as well as chromatic dispersion that, for the wavelengths situated in the transmission window around 1550 nm, is both low enough not to give rise to information losses during transmission, and also high enough to avoid the four-wave mixing phenomenon over the entire transmission window. In addition, the chromatic dispersion gradient of the fiber of the invention is shallow.
Advantageously, the fiber is single-mode in fiber.
For high data rate systems, it is also useful for the fiber to provide single-mode propagation of the channels of the multiplex. ITU-T G 650 gives a definition of the cutoff wavelength in cable. The theoretical cutoff wavelength of the fiber is generally greater by several hundred nanometers than the cutoff wavelength in cable. The propagation in an optical fiber can be single-mode even if the theoretical cutoff wavelength is greater than the wavelength of the signals used: beyond a distance of a few meters or a few tens of meters, which is short compared with the propagation distances in optical-fiber transmission systems, the secondary modes disappear because of attenuation that is too great. The propagation in the transmission system is then single-mode.
In addition, the bending losses of the fiber of the invention are less than 0.05 dB at 1550 nm for a winding of 100 turns of the fiber around a radius of 30 mm.
Advantageously, the fiber has chromatic dispersion lying, in absolute terms, in the range 3 ps/nm.km to 6 ps/nm.km for a wavelength of 1550 nm.
The fiber has a chromatic dispersion gradient lying, in absolute terms, in the range 0.045 ps/nm2.km to 0.09 ps/nm2.km, and preferably in the range 0.045 ps/nm2.km to 0.075 ps/nm2.km for wavelengths of 1550 nm. These values for the chromatic dispersion gradient guarantee, in the range in which the fiber is used, that the chromatic dispersion remains substantially constant. The fiber is thus suitable for use under wavelength division multiplexing, and has chromatic dispersion values that are substantially equal over the band of the multiplex.
Preferably, the attenuation of the fiber is less than or equal to 0.23 dB/km at 1550 nm. Such an attenuation value guarantees that transmission losses are limited.
In a first embodiment of the invention, the fiber has an index profile comprising a central portion of index ni less than the index ns of the cladding of the fiber, an inner ring of index n2 greater than the index of the cladding and extending around said central portion, an outer ring of index n4 greater than the index of the cladding and extending around said inner ring, and, between said inner ring and said outer ring, an annular portion of index n3 less than or equal to the indices of said inner ring and of said outer ring.
In which case, the difference xcex94n1 between the index of the central portion of the fiber and the index of the cladding preferably lies within a range of xe2x88x926.5xc3x9710xe2x88x923xc2x110%.
The radius a1 of the central portion of the fiber advantageously lies within a range of 2.1 xcexcmxc2x110%.
In an embodiment, the difference xcex94n2 between the index of the inner ring of the fiber and the index of the cladding lies within a range of 12.0xc3x9710xe2x88x923xc2x110%.
It is possible to make provision for the ratio a1/a2 between the radius a1 of the central portion of the fiber and the radius a2 of the inner ring to lie within a range of 0.50xc2x110%.
In another embodiment, the difference xcex94n3 between the index of the annular portion and the index of the cladding lies within a range of xe2x88x926.5xc3x9710xe2x88x923xc2x110%.
In addition, the thickness a3xe2x88x92a2 of the annular portion advantageously lies within a range of 2.1 xcexcmxc2x110%.
Preferably, the difference xcex94n4 between the index of the outer ring and the index of the cladding lies within a range of 3.5xc3x9710xe2x88x923xc2x110%.
In addition, the thickness a4xe2x88x92a3 of the outer ring lies within a range of 2.1 xcexcmxc2x110%.
In a second embodiment of the invention, the fiber has an index profile comprising a central portion of maximum index n1 greater than the index ns of the optical cladding, a ring of maximum index n4 greater than the index of the optical cladding and surrounding said central portion, and an intermediate zone of index n2 less than n1, and less than n4, between said central portion and said ring.
In a preferred implementation of the second embodiment, the difference xcex94n1 between n1 and ns lies in the range 9xc3x9710xe2x88x923 to 17xc3x9710xe2x88x923.
The difference xcex94n4 between n4 and ns is less than 12xc3x9710xe2x88x923.